Directly after polymerization fluoropolymers have among their endgroups certain types that are designated as unstable. Unstable endgroups may decompose or otherwise react at the temperatures at which the fluoropolymers are melt-processed, that is, they are thermally unstable. They can cause bubbles or voids in the processed fluoropolymer. They may also be a source of hydrogen fluoride in the finished articles of melt-processing, for example by exposure of the articles to atmospheric moisture. That is, they are hydrolytically unstable. Among these unstable endgroups are: —COOH, which can decarboxylate at processing temperatures, generating carbon dioxide, a potential source of bubbles and voids; —COF, which though more thermally stable, hydrolyzes easily to —COOH and HF; —CF═CF2, which oxidizes to —COF; and —CONH2, which although desirable in certain applications due to its greater thermal stability than —COOH ends and greater hydrolytic stability than —COF ends can react or decompose into —COOH, —COF, or —CF═CF2 groups, often with color formation.
Fluorination is among the ways to reduce the number of unstable endgroups in fluoropolymers that are disclosed. Such fluorination converts the non-perfluorinated unstable end groups to the highly stable perfluorinated end group —CF3. British Patent 1,210,794 describes contacting solid fluoropolymer with fluorine. Because of the solid and gas phase nature of this system, particle size and efficiency of contact between the gas and the solid will affect the time needed to achieve unstable endgroup reduction. Contact times on the order of hours are typical of fluorination of solid polymer.
U.S. Pat. No. 4,626,587 discloses that as-polymerized melt-processible fluoropolymer such as tetrafluoroethylene (TFE)/hexafluoropropylene (HFP) copolymer (FEP), has an additional source of instability, namely relatively unstable linkages in the polymer backbone arising from adjacent HFP units in the polymer backbone. This patent teaches removal of this instability by subjecting the fluoropolymer to high shear in a twin-screw extruder, the twin screws preferably containing kneading blocks. The unstable end groups of the fluoropolymer and/or any poor (dark) color present in the fluoropolymer after this high shear treatment are eliminated by an after-treatment of the fluoropolymer, such as fluorination as described in British Patent 1,210,794 or humid heat treatment such as disclosed in U.S. Pat. No. 3,085,083, wherein water (steam) converts —CF2COOH end groups to more stable —CF2H end groups. Fluorination of the extruded cubes of fluoropolymer changes the dark as-extruded color of the cubes to white. U.S. '587 also discloses the addition of 1 wt % water to the twin-screw extruder to hydrolyze acid fluoride end groups (paragraph bridging cols. 7 and 8), this amount of water corresponding to a molar excess of 751× with respect to the 37 acid fluoride groups present/106 carbon atoms when the water was not added.
European Patent Application 0 928 673 A1 (WO98/09784) discloses the removal of unstable end groups, unstable bonds in polymer backbones of fluoropolymers, and poor color from the fluoropolymer by subjecting the fluoropolymer to a melt kneading step. This publication observes that the use of a twin-screw extruder in U.S. '587 provides too short a residence time and requires the after-treatment to remove unstable end groups and poor color and overcomes this shortcoming by using a kneader, which has a higher usable volume ratio (usable space/actual space) than a twin-screw extruder and carries out the kneading of the molten fluoropolymer using paddles mounted upon a shaft for at least 10 minutes, 40 to 60 min. being used in the Examples in the presence of fluorine. During this melt-kneading, the molten polymer is exposed to reactants which are disclosed to achieve the stabilization and decolorizing effects, such reactants being at least one of the following: fluorine, water or steam, salts or bases or alcohol.
European Patent Publication 1 170 303 A1 criticizes the melt kneading of WO 98/09784 as promoting the depolymerization of the perfluorovinyl end groups and deterioration of the fluoropolymer. EP '303 also criticizes the large size and long time required by a surface renewal type kneader. EP '303 addresses this problem by melt kneading the fluoropolymer in a twin-screw extruder equipped with a kneading block in a treatment zone within the extruder for a period of time of 0.2 to 5 min, 2 min. being used in the Examples. Both water and oxygen are the reactants added to the treatment zone, the omission of oxygen giving a poor result as indicated in Comparative Example 1. Preferably a salt, potassium carbonate is exemplified, is also present in the treatment zone. The amount of oxygen present is at least stoichiometric with respect to the perfluorovinyl groups present, preferably an excess amount, disclosing at least 10 moles of oxygen per mole of perfluorovinyl group, and particularly a molar excess of 50 to 500. The molar amount of water present can be the same number of molecules as the unstable end groups to be stabilized, but preferably an excess amount, particularly at least 10× excess amount. To accomplish the stabilization reported in Example 1 with 2 min. of melt kneading at superatmospheric pressure, the molar excess of water and oxygen relative to —COOH ends calculate to 1337× and 198×, respectively. To accomplish the stabilization reported in Example 2 at subatmospheric pressure, the molar excess of water and oxygen calculate to 853× and 79×, respectively. The stabilization goal as reported in Tables 1 and 2 is to convert —COOH end groups to —CF2H.
EP '303 also discloses fluorine treatment of the fluoropolymer after the stabilization treatment of the invention of (p. 5, I. 17–19), i.e. after the stabilized fluoropolymer exits the twin-screw extruder. Fluorination provides the end groups of greatest stability, namely —CF3.
More efficient and more effective processing of fluoropolymer to change its character, e.g. to stabilize the fluoropolymer, is needed.